It's the same with the LK chests. They can pop out high runes, but like all the high runes, EU's and valuable set items, they take a LOT of runs to get.
The people finding these are running the areas very quickly, and having a lot of runs.

If they do find a HR without running a lot, then it'd be luck, but I think they just run the chests frequently.

The Meph Chest and LK runs are mentioned all the time ... because people run them all the time. That's why you hear about it.

People always pop the Meph chest (like me) because it's there. Only takes a moment so what the heck. I don't think i've ever gotten a decent rune from that chest ... much less a unique ... yet I pop it anyways. LK chest are great because you can pop tons of flawless as well as charms and runes and the occasional unique (like a Stormshield, Skullder's Ire etc ...)

Now, take me for example. I've found 4 high runes. 3 Lo's and a Sur. One Lo came from popping a bucket on Durance 2 while I was trying to get to the Durance 3 stairs. Another one from a boss pack outside the Countess entrace on players1. The Sur was from LK but not from a superchest ... you get the picture.

They popped randomly ... and for all the time and effort i've put into this game i'm rewarded eventually for my time put in.